1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, is concerned with an attachment structure useful for attaching turbine blades at their outer ends to a rotor of a gas turbine engine.
2. Description of the Prior Art
Gas turbine engines generally include a gas generator which comprises a compressor for compressing air flowing aft through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a turbine driven by the gas stream which is connected by a shaft to, in turn, drive the compressor. Many engines further include a second turbine, known as a power turbine, located aft of the gas generator and which extracts energy from the gas flow to drive a rotating load such as found in the propulsor of helicopters and turboprop engines and fixed pitch blades in ducted turbofan engines.
A recent improvement over the turbofan and turboprop engines is an unducted fan engine such as disclosed in the first U.S. patent application cross-referenced above. In the unducted fan engine, the power turbine includes counterrotating rotors with turbine blades defining counterrotating airfoil stages which drive in corresponding fashion unducted fan blades located radially outwardly with respect to the power turbine. The fan blades of the unducted fan engine are variable pitch blades to achieve optimum performance under varying operating conditions.
In prior art turbine engines, dovetail structures have been widely accepted as efficient means of attaching turbine blades to structural components, such as a turbine rotor. However, in view of the relatively low rotational speeds of the counterrotating rotors in the power turbine of the unducted fan engine, more flexibility is allowed in the design of turbine blade attachment structures to meet restrictive weight requirements. Further, the attachment of the blades at the radially outer end rather than the radially inner end requires a different structure.
Consequently, a need exists for alternative approaches which take advantage of this added flexibility to provide lightweight structures to accommodate the attachment of turbine blades to the outer casing of a counterrotating rotor in turbomachinery applications.